magento checkout page error with exception log

Question

the magento checkout page give an error saying
There was an error processing your order
I am getting this error. It was working fine from six months. I haven't changed anything.

Troubleshoot:

I have changed to Magento default theme.
I have changed the domain
Change checkout method
Still not working.

my exception.log

2017-04-17T17:28:18+00:00 ERR (3): 
exception 'Mage_Core_Exception' with message 'Invalid block type: Cmsmart_Onestepcheckout_Block_Links' in /home/zprz/public_html/app/Mage.php:595
Stack trace:
#0 /home/zprz/public_html/app/code/core/Mage/Core/Model/Layout.php(495): Mage::throwException('Invalid block t...')
#1 /home/zprz/public_html/app/code/core/Mage/Core/Model/Layout.php(437): Mage_Core_Model_Layout->_getBlockInstance('onestepcheckout...', Array)
#2 /home/zprz/public_html/app/code/core/Mage/Core/Model/Layout.php(472): Mage_Core_Model_Layout->createBlock('onestepcheckout...', 'checkout_cart_l...')
#3 /home/zprz/public_html/app/code/core/Mage/Core/Model/Layout.php(239): Mage_Core_Model_Layout->addBlock('onestepcheckout...', 'checkout_cart_l...')
#4 /home/zprz/public_html/app/code/core/Mage/Core/Model/Layout.php(205): Mage_Core_Model_Layout->_generateBlock(Object(Mage_Core_Model_Layout_Element), Object(Mage_Core_Model_Layout_Element))
#5 /home/zprz/public_html/app/code/core/Mage/Core/Model/Layout.php(210): Mage_Core_Model_Layout->generateBlocks(Object(Mage_Core_Model_Layout_Element))
#6 /home/zprz/public_html/app/code/core/Mage/Core/Controller/Varien/Action.php(344): Mage_Core_Model_Layout->generateBlocks()
#7 /home/zprz/public_html/app/code/core/Mage/Cms/Helper/Page.php(113): Mage_Core_Controller_Varien_Action->generateLayoutBlocks()
#8 /home/zprz/public_html/app/code/core/Mage/Cms/Helper/Page.php(52): Mage_Cms_Helper_Page->_renderPage(Object(Mage_Cms_IndexController), 'no-route')
#9 /home/zprz/public_html/app/code/core/Mage/Cms/controllers/IndexController.php(75): Mage_Cms_Helper_Page->renderPage(Object(Mage_Cms_IndexController), 'no-route')
#10 /home/zprz/public_html/app/code/core/Mage/Core/Controller/Varien/Action.php(418): Mage_Cms_IndexController->noRouteAction()
#11 /home/zprz/public_html/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(254): Mage_Core_Controller_Varien_Action->dispatch('noRoute')
#12 /home/zprz/public_html/app/code/local/Elegento/Royal/Controller/Standard.php(82): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#13 /home/zprz/public_html/app/code/core/Mage/Core/Controller/Varien/Front.php(172): Elegento_Royal_Controller_Standard->match(Object(Mage_Core_Controller_Request_Http))
#14 /home/zprz/public_html/app/code/core/Mage/Core/Model/App.php(365): Mage_Core_Controller_Varien_Front->dispatch()
#15 /home/zprz/public_html/app/Mage.php(684): Mage_Core_Model_App->run(Array)
#16 /home/zprz/public_html/index.php(83): Mage::run('', 'store')
#17 {main}
2017-04-17T17:28:18+00:00 ERR (3): 
exception 'PDOException' with message 'SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`zprz_mag`.`erp_inventory_warehouse_product`, CONSTRAINT `erp_inventory_warehouse_product_ibfk_1` FOREIGN KEY (`warehouse_id`) REFERENCES `erp_inventory_warehouse` (`warehouse_id`) ON DELETE C)' in /home/zprz/public_html/lib/Zend/Db/Statement/Pdo.php:228
Stack trace:
#0 /home/zprz/public_html/lib/Zend/Db/Statement/Pdo.php(228): PDOStatement->execute(Array)
#1 /home/zprz/public_html/lib/Varien/Db/Statement/Pdo/Mysql.php(110): Zend_Db_Statement_Pdo->_execute(Array)
#2 /home/zprz/public_html/app/code/core/Zend/Db/Statement.php(291): Varien_Db_Statement_Pdo_Mysql->_execute(Array)
#3 /home/zprz/public_html/lib/Zend/Db/Adapter/Abstract.php(480): Zend_Db_Statement->execute(Array)
#4 /home/zprz/public_html/lib/Zend/Db/Adapter/Pdo/Abstract.php(238): Zend_Db_Adapter_Abstract->query('INSERT INTO `er...', Array)
#5 /home/zprz/public_html/lib/Varien/Db/Adapter/Pdo/Mysql.php(504): Zend_Db_Adapter_Pdo_Abstract->query('INSERT INTO `er...', Array)
#6 /home/zprz/public_html/lib/Zend/Db/Adapter/Abstract.php(576): Varien_Db_Adapter_Pdo_Mysql->query('INSERT INTO `er...', Array)
#7 /home/zprz/public_html/app/code/core/Mage/Core/Model/Resource/Db/Abstract.php(453): Zend_Db_Adapter_Abstract->insert('erp_inventory_w...', Array)
#8 /home/zprz/public_html/app/code/core/Mage/Core/Model/Abstract.php(318): Mage_Core_Model_Resource_Db_Abstract->save(Object(Magestore_Inventory_Model_Warehouseproduct))
#9 /home/zprz/public_html/app/code/local/Magestore/Inventory/Model/Observer.php(1405): Mage_Core_Model_Abstract->save()
#10 /home/zprz/public_html/app/code/core/Mage/Core/Model/App.php(1357): Magestore_Inventory_Model_Observer->salesOrderPlaceAfter(Object(Varien_Event_Observer))
#11 /home/zprz/public_html/app/code/core/Mage/Core/Model/App.php(1336): Mage_Core_Model_App->_callObserverMethod(Object(Magestore_Inventory_Model_Observer), 'salesOrderPlace...', Object(Varien_Event_Observer))
#12 /home/zprz/public_html/app/Mage.php(448): Mage_Core_Model_App->dispatchEvent('sales_order_pla...', Array)
#13 /home/zprz/public_html/app/code/core/Mage/Sales/Model/Order.php(1117): Mage::dispatchEvent('sales_order_pla...', Array)
#14 [internal function]: Mage_Sales_Model_Order->place()
#15 /home/zprz/public_html/app/code/core/Mage/Core/Model/Resource/Transaction.php(105): call_user_func(Array)
#16 /home/zprz/public_html/app/code/core/Mage/Core/Model/Resource/Transaction.php(159): Mage_Core_Model_Resource_Transaction->_runCallbacks()
#17 /home/zprz/public_html/app/code/core/Mage/Sales/Model/Service/Quote.php(189): Mage_Core_Model_Resource_Transaction->save()
#18 /home/zprz/public_html/app/code/core/Mage/Sales/Model/Service/Quote.php(249): Mage_Sales_Model_Service_Quote->submitOrder()
#19 /home/zprz/public_html/app/code/core/Mage/Checkout/Model/Type/Onepage.php(812): Mage_Sales_Model_Service_Quote->submitAll()
#20 /home/zprz/public_html/app/code/core/Mage/Checkout/controllers/OnepageController.php(579): Mage_Checkout_Model_Type_Onepage->saveOrder()
#21 /home/zprz/public_html/app/code/core/Mage/Core/Controller/Varien/Action.php(418): Mage_Checkout_OnepageController->saveOrderAction()
#22 /home/zprz/public_html/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(254): Mage_Core_Controller_Varien_Action->dispatch('saveOrder')
#23 /home/zprz/public_html/app/code/local/Elegento/Royal/Controller/Standard.php(82): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#24 /home/zprz/public_html/app/code/core/Mage/Core/Controller/Varien/Front.php(172): Elegento_Royal_Controller_Standard->match(Object(Mage_Core_Controller_Request_Http))
#25 /home/zprz/public_html/app/code/core/Mage/Core/Model/App.php(365): Mage_Core_Controller_Varien_Front->dispatch()
#26 /home/zprz/public_html/app/Mage.php(684): Mage_Core_Model_App->run(Array)
#27 /home/zprz/public_html/index.php(83): Mage::run('', 'store')
#28 {main}

Answer 1

You have problem with the warehouse plugin (most probably magestore inventory). During the insert in the database there is something wrong that happens. Not knowing so much about the module or the environment, I just can tell you that the request tries to create or update a dependent table but does not succeed. The reasons are multiple but most probably the row alread exists.